Non-volatile memory cells are well known in the art. One prior art non-volatile split gate memory cell 10, which contains five terminals, is shown in FIG. 1. Memory cell 10 comprises semiconductor substrate 12 of a first conductivity type, such as P type. Substrate 12 has a surface on which there is formed a first region 14 (also known as the source line SL) of a second conductivity type, such as N type. A second region 16 (also known as the drain line) also of N type is formed on the surface of substrate 12. Between the first region 14 and the second region 16 is channel region 18. Bit line BL 20 is connected to the second region 16. Word line WL 22 is positioned above a first portion of the channel region 18 and is insulated therefrom. Word line 22 has little or no overlap with the second region 16. Floating gate FG 24 is over another portion of channel region 18. Floating gate 24 is insulated therefrom, and is adjacent to word line 22. Floating gate 24 is also adjacent to the first region 14. Floating gate 24 may overlap the first region 14 to provide coupling from the first region 14 into floating gate 24. Coupling gate CG (also known as control gate) 26 is over floating gate 24 and is insulated therefrom. Erase gate EG 28 is over the first region 14 and is adjacent to floating gate 24 and coupling gate 26 and is insulated therefrom. The top corner of floating gate 24 may point toward the inside corner of the T-shaped erase gate 28 to enhance erase efficiency. Erase gate 28 is also insulated from the first region 14. Memory cell 10 is more particularly described in U.S. Pat. No. 7,868,375, whose disclosure is incorporated herein by reference in its entirety.
One exemplary operation for erase and program of prior art non-volatile memory cell 10 is as follows. Memory cell 10 is erased, through a Fowler-Nordheim tunneling mechanism, by applying a high voltage on erase gate 28 with other terminals equal to zero volt. Electrons tunnel from floating gate 24 into erase gate 28 causing floating gate 24 to be positively charged, turning on the cell 10 in a read condition. The resulting cell erased state is known as ‘1’ state.
Memory cell 10 is programmed, through a source side hot electron programming mechanism, by applying a high voltage on coupling gate 26, a high voltage on source line 14, a medium voltage on erase gate 28, and a programming current on bit line 20. A portion of electrons flowing across the gap between word line 22 and floating gate 24 acquire enough energy to inject into floating gate 24 causing the floating gate 24 to be negatively charged, turning off the cell 10 in a read condition. The resulting cell programmed state is known as ‘0’ state.
Memory cell 10 is read in a Current Sensing Mode as following: A bias voltage is applied on bit line 20, a bias voltage is applied on word line 22, a bias voltage is applied on coupling gate 26, a bias or zero voltage is applied on erase gate 28, and a ground is applied on source line 14. There exists a cell current flowing from bit line 20 to source line 14 for an erased state and there is insignificant or zero cell current flow from the bit line 20 to the source line 14 for a programmed state. Alternatively, memory cell 10 can be read in a Reverse Current Sensing Mode, in which bit line 20 is grounded and a bias voltage is applied on source line 24. In this mode the current reverses the direction from source line 14 to bitline 20.
Memory cell 10 alternatively can be read in a Voltage Sensing Mode as following: A bias current (to ground) is applied on bit line 20, a bias voltage is applied on word line 22, a bias voltage is applied on coupling gate 26, a bias voltage is applied on erase gate 28, and a bias voltage is applied on source line 14. There exists a cell output voltage (significantly >0V) on bit line 20 for an erased state and there is insignificant or close to zero output voltage on bit line 20 for a programmed state. Alternatively, memory cell 10 can be read in a Reverse Voltage Sensing Mode, in which bit line 20 is biased at a bias voltage and a bias current (to ground) is applied on source line 14. In this mode, memory cell 10 output voltage is on the source line 14 instead of on the bit line 20.
In the prior art, various combinations of positive or zero voltages were applied to word line 22, coupling gate 26, and floating gate 24 to perform read, program, and erase operations
In response to the read, erase or program command, the logic circuit 270 (in FIG. 2) causes the various voltages to be supplied in a timely and least disturb manner to the various portions of both the selected memory cell 10 and the unselected memory cells 10.
For the selected and unselected memory cell 10, the voltage and current applied are as follows. As used hereinafter, the following abbreviations are used: source line or first region 14 (SL), bit line 20 (BL), word line 22 (WL), and coupling gate 26 (CG).
TABLE NO. 1PEO (Positive Erase Operation) TableWL-BL-CG-unselCG-EG-WLunselBLunselCGsame sectorunselEGunselSLSL-unselRead1.0-2 V0 V0.6-2 V0 V-FLT0-2.6V0-2.6 V0-2.6 V0-2.6 V0-2.6 V0 V0 V-FLTErase0 V0 V0 V0 V0 V0-2.6 V0-2.6 V11.5-12 V0-2.6 V0 V0 VProgram1V0 V1 uAVinh10-11 V0-2.6 V0-2.6 V4.5-5 V0-2.6 V4.5-5V0-1 V/FLT
In a recent application by the applicant—U.S. patent application Ser. No. 14/602,262, filed on Jan. 21, 2015, which is incorporated by reference—the applicant disclosed an invention whereby negative voltages could be applied to word line 22 and/or coupling gate 26 during read, program, and/or erase operations. In this embodiment, the voltage and current applied to the selected and unselected memory cell 10, are as follows.
TABLE NO. 2PEO (Positive Erase Operation) TableWL-BL-CG-unselCG-EG-WLunselBLunselCGsame sectorunselEGunselSLSL-unselRead1.0-2 V−0.5 V/0 V0.6-2 V0 V-FLT0-2.6V0-2.6 V0-2.6 V0-2.6 V0-2.6 V0 V0 V-FLTErase0 V0 V0 V0 V0 V0-2.6 V0-2.6 V11.5-12 V0-2.6 V0 V0 VProgram1V−0.5 V/0 V1 uAVinh10-11 V0-2.6 V0-2.6 V4.5-5 V0-2.6 V4.5-5V0-1 V/FLT
In another embodiment of U.S. patent application Ser. No. 14/602,262, negative voltages can be applied to word line 22 when memory cell 10 is unselected during read, erase, and program operations, and negative voltages can be applied to coupling gate 26 during an erase operation, such that the following voltages are applied:
TABLE NO. 3PNEO (Positive Negative Erase Operation) TableWL-BL-CG-unselCG-EG-WLunselBLunselCGsame sectorunselEGunselSLSL-unselRead1.0-2 V−0.5 V/0 V0.6-2 V0-FLT0-2.6V0-2.6 V0-2.6 V0-2.6 V0-2.6 V0 V0-FLTErase0 V−0.5 V/0 V0 V0-FLT−(5-9) V0-2.6 V0-2.6 V8-9V0-2.6 V0 V0 VProgram1V−0.5 V/0 V1 uAVinh8-9 VCGINH 0-2.6 V8-9 V0-2.6 V4.5-5V0-1 V/FLT(4-6 V)
The CGINH signal listed above is an inhibit signal that is applied to the coupling gate 26 of an unselected cell that shares an erase gate 28 with a selected cell.
FIG. 2 depicts an embodiment recently developed by applicant of an architecture for a flash memory system comprising die 200. Die 200 comprises: memory array 215 and memory array 220 for storing data, memory arrays 215 and 220 comprising rows and columns of memory cells of the type described previously as memory cell 10 in FIG. 1, pad 240 and pad 280 for enabling electrical communication between the other components of die 200 and, typically, wire bonds (not shown) that in turn connect to pins (not shown) or package bumps that are used to access the integrated circuit from outside of the packaged chip or macro interface pins (not shown) for interconnecting to other macros on a SOC (system on chip); high voltage circuit 275 used to provide positive and negative voltage supplies for the system; control logic 270 for providing various control functions, such as redundancy and built-in self-testing; analog circuit 265; sensing circuits 260 and 261 used to read data from memory array 215 and memory array 220, respectively; row decoder circuit 245 and row decoder circuit 246 used to access the row in memory array 215 and memory array 220, respectively, to be read from or written to; column decoder circuit 255 and column decoder circuit 256 used to access bytes in memory array 215 and memory array 220, respectively, to be read from or written to; charge pump circuit 250 and charge pump circuit 251, used to provide increased voltages for program and erase operations for memory array 215 and memory array 220, respectively; negative voltage driver circuit 230 shared by memory array 215 and memory array 220 for read and write operations; high voltage driver circuit 225 used by memory array 215 during read and write operations and high voltage driver circuit 226 used by memory array 220 during read and write operations.
With flash memory systems becoming ubiquitous in all manner of computing and electronic devices, it is increasingly important to create designs that reduce the amount of die space required per memory cell and to reduce the overall complexity of decoders use in flash memory systems. What is needed is flash memory cell design that utilizes fewer terminals than in the prior art and simplified circuitry for operating flash memory cells that follow that design.